Phase interpolators are used to produce a varying output clock phase for a given fixed-phase input clock. Phase interpolators (also referred to as “phase rotators”) can “rotate” or “step” the output clock phase anywhere (in discrete steps) between the edges that define a period of the input clock. Phase interpolators are often used in clock recovery operations.
It is often desirable for the phase step size of a phase interpolator to remain constant over every possible output phase location of the phase interpolator. For example, a constant step size in a phase interpolator can improve clock recovery jitter performance, improve interphase clock skews, and improve clock skew matching between multiple independent phase interpolators. Differential Non-Linearity (DNL) is a measure of the variation between any two adjacent phase interpolator steps. An ideal phase interpolator would have zero DNL (i.e., no variation). Integral Non-Linearity (INL) is a measure of the DNL integrated over N steps. In this way, INL shows how far the phase of a non-ideal phase interpolator deviates from the phase of an ideal phase interpolator over N steps.
Many conventional phase interpolator architectures suffer from varying degrees of DNL, which can lead to INL-induced phase differences between separate phase interpolators that are desired to match. The DNL of traditional phase interpolator architectures can also lead to rotational I-Q clock skews within any individual phase interpolator. Further, mixed waveforms are not perfectly sinusoidal, which leads to step size errors, and this issue can be exacerbated by digital-to-analog converter (DAC) current mirror errors that have voltage modulation on their drains as well as differential pairs that exhibit nonlinear behavior.
These conventional phase interpolators suffer from a “small-step/large-step” INL issue, where the phase interpolator takes large steps at one end of the rotation quadrant to make up for small steps at the opposite end of the rotation quadrant. Phase blending output buffers may be used to partially average out the effects of interphase skews in a phase interpolator. However, use of these phase blending output buffers does not improve INL errors between matched interpolators due to the “small steps/large steps” problem.